An electrophotographic color image forming apparatus has independent image forming parts for forming yellow, magenta, cyan, and black images. The images of these colors are consecutively transferred from the image forming parts to an intermediate transfer belt, and then the images are collectively transferred from the intermediate transfer belt to a recording media. Thus, the electrophotographic color image forming apparatus achieves high speed printing.
The image forming parts for these colors each include a photosensitive drum, which serves as an image bearing member, a charging member, which charges the photosensitive drum, and a developing unit, which develops a toner image onto the photosensitive drum. The charging member of each image forming part comes into contact with the corresponding photosensitive drum at a predetermined pressure, and charges the surface of the photosensitive drum with a charging voltage applied from a charging voltage source in such a manner that the surface uniformly has a predetermined potential with a predetermined polarity.
The developing unit of each image forming part attaches toner to an electrostatic latent image formed on the corresponding photosensitive drum and then develops the latent image into a toner image (visible image).
Toner images developed onto the photosensitive drums of the image forming parts are first-transferred to the intermediate transfer belt by first transfer rollers, which are opposite the photosensitive drums with the intermediate transfer belt being interposed therebetween and which serve as first transfer portions. The first transfer rollers are connected to corresponding first-transfer voltage sources.
The toner images that have been first-transferred to the intermediate transfer belt are then second-transferred to a transfer medium by a second transfer unit. A second transfer roller that serves as the second transfer unit is connected to a second-transfer voltage source.
PTL 1 discloses an apparatus that includes four first transfer rollers connected to four corresponding first-transfer voltage sources. PTL 2 discloses an apparatus that performs control so that, before an image forming operation, a transfer voltage to be applied to each first transfer roller is changed in accordance with the properties of the intermediate transfer belt and the first transfer roller, including a sheet-feeding durability and a resistance that varies due to environmental changes.
PTL 3 discloses an image forming apparatus of a known type in which a residual toner remaining on an intermediate transfer belt is charged by a charging member and then transferred to an image forming part in a first transfer portion to be recovered. In this structure, the image forming part recovers the residual toner on the intermediate transfer belt. Thus, the need for a waste-toner container dedicated to the intermediate transfer belt is eliminated, and a compact apparatus is achieved, accordingly.